Ashrae Journal - November 2008 - (Page 32)

Use Less Glass, Use Good Glass The results of a number of scenarios are plotted in Figure 2. Typical curtain wall systems have an R-value of only 2 or 3, with “high performance” systems (not shown) using highly insulated spandrel panels and best-inclass double glazing may achieve R4. Only a few systems, such as the Kawneer 7550 series, can achieve R-values of 6 or more. Curve 1 is for standard U=0.50 thermally broken aluminum punched windows with air-filled double-glazed insulated glazing units in a R12 batt-filled steel-stud brick veneer wall system (R6). The overall effective R-value of this wall is around 3 to 4 over the normal range Figure 2: Enclosure R-Value Versus Glazing Ratio. Bottom line is use less glass and of window-to-wall (WWR) ratios of use good glass. The chart is courtesy of John Straube.6 Bad glass ruins good walls. The impact of thermal bridging through commercial wall assemblies, and heat ἀow through 25% to 50%. Curve 2 shows that increasing the window systems can be calculated with relatively good accuracy by calculating an areaR-value of the wall to R11 by adding weighted average of the R-values of the windows and opaque wall sections. The equation an inch of foam on the exterior, results takes the form: Uoverall = (WWR · Uwindow + (1 – WWR) · Uwall), where U = 1/R. in an increase of only R0.5 to R1.5 for overall R-value will be in the 7 to 12 range for WWR of the overall R-value for the same range of WWR. Curve 3 shows how significant an impact window per- less than 40% (the highest ratio recommended for highformance can make if a good wall is provided. An externally performance buildings). Even though this is a low-level, insulated R16 wall, when mated with poor windows produces it is still significantly more than the alternative. The grey a vertical enclosure with an R-value of only R3 to R6 for the curve below Curve 5 shows the slight benefit gained by increasing wall R-value from 20 to 40, particularly at high normal range of window area. Curve 4 assumes a good quality window frame with top glazing ratios. Curve 6 uses low-e, argon-filled triple-glazed units quality glazing (low-e, argon-filled): the result for the overall in an insulated fiberglass frame, to deliver a U-value of vertical enclosure is still only R4 to R7. These first four curves cover the performance of a wide only 0.14. Even with a wall insulated to “just” R20, such range of commercial enclosures with a wide range of cladding a combination can deliver an overall R-value of 12 to 14, types. The conclusion is that modern commercial vertical two to three times more than typical commercial vertical enclosures actually have an R-value that is rarely over 7, and enclosures. In all cases, it can be seen that high glazing ratios generate more likely in the range of 3 to 5! Curves 5 and 6 provide an idea of the significant im- enclosure walls that are expensive to purchase with high heat provements that are possible. Using best-in-class thermally loss and heat gain. This high ratio should be avoided in both broken aluminum frames and high-performance glazing individual spaces, such as meeting rooms, as for the whole (U=0.30), Curve 5 shows that even with an R40 wall, the building on average. decent façade with less than 100% glass. Why even go there? Oh, I forgot about all the passive ventilation “magic” that happens between the two facades and the operable windows you can have between the inner façade and the “magic” space. All brought to you with the precision and predictability of computational fluid dynamics (CFD) and the stack effect. Emswiler2 and Hutcheon3 are rolling over in their graves and Shaw and Tamura4 are none too pleased. I call on the ghosts of building science past to rise up and put a pox on all your houses. 32 ASHRAE Journal I have news for all you façadists. You can have operable windows in a single façade, and you can get a lot more control and predictability with fans, ductwork and controls. You can get it at much less cost, using much less materials (i.e., resource efficiency) and much less energy. Yes, fans use energy. It’s not natural to use fans. The other way, the “magic” way, uses “natural” forces, which is good because nature is good and man is inherently evil. Didn’t we have this argument over a hundred years ago with a dead French guy called Rousseau? If we taught ashrae.org November 2008 http://www.ashrae.org

Table of Contents Feed for the Digital Edition of Ashrae Journal - November 2008

Ashrae Journal - November 2008 Contents Commentary Industry News Meetings and Shows Feature Articles Reducing Data Center Energy Consumption Building Sciences Feature Articles Sustainable Nature Center Emerging Technologies Special Products Washington Report Special Supplement: BACnet® Today BACnet® Goes Green Seeing the Light with BACnet® BACnet® on Campus BACnet® for Net Zero Analyzing BACnet® BACnet® at Xerox Campus BACnet® in Europe BACnet® for Green Library BACnet® at the University of Minnesota: Flexibility, Interoperability InfoCenter Product Showplace Classified Advertising Advertising Index

Ashrae Journal - November 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.